This invention relates to water-based printing inks; and more particularly to fluorescent ink compositions useful in various printing~methods such as ink jet printing.
Ink jet printing is a standard and preferred method for printing onto a substrate, wherein a stream of ink droplets are directed from a printing device to a surface of the substrate. The direction of the stream is controlled electronically in causing the droplets to print the desired image or information on the substrate surface without requiring contact between the printing device and the surface to which the ink is applied. Ink jet printing may be applied to a variety of substrates including, but not limited to, metals, glass, synthetic resins, plastics, rubber, paper, and the like. Objects, comprising substrates to which ink jet printing is well suited, include, but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, bank cards (debit, credit, and the like), identification strips (e.g., comprising barcodes), and the like.
Fluorescent inks have been developed for printing a xe2x80x9csecurity markxe2x80x9d on articles such that the mark is invisible to the unaided eye, but that can be detected as fluorescence upon excitation with an activating light of a suitable excitation wavelength spectrum. More particularly, security marks are applied to articles in efforts to prevent forgery, theft, and fraud; wherein such articles are known to include, but are not limited to, identification cards, passports, currency, checks, securities, and other types of commercial paper. The security mark may be in the form of a barcode which encodes information, or may comprise a recognizable pattern for identification and verification purposes. Prior art fluorescent inks are known in the art to include: a fluorescent colorant, a solvent, and a binder resin; an organic laser dye that is excited by a wavelength spectrum in the near infrared range and fluoresces in an infrared range; a phenoxazine derivative dye fluorescing in the near infrared range; a rare earth metal, an organic ink carrier, and may further comprise a chelating agent; and a near infrared fluorophore and a water-dissipatable polyester. Generally, such fluorescent inks comprising fluorescent dyes or pigments, present with several disadvantages. For example, there is a relatively narrow margin between the amount of a fluorescent dye which is sufficient to give good fluorescent color intensity, and an amount wherein the dye molecules begin to aggregate and thus reduce the amount of fluorescence by quenching. A limitation in intensity can also limit the density of information encoded on a security mark (such as a barcode) by a fluorescent ink composition.
Thus, there exists a need for fluorescent ink compositions suitable for printing on substrates, wherein (a) the fluorescent component of the fluorescent ink composition is water-soluble; (b) the fluorescent component is excited by a wavelength spectrum comprising UV light, and preferably in a spectral range of from about 300 nanometers (nm) to about 400 nm, and emits an narrow emission peak in a wavelength spectrum primarily in the visible range, and preferably in a spectral range of from about 410 nm to about 750 nm; (c) a plurality of fluorescent ink compositions (each containing a fluorescent component that can be detectably distinguished (e.g., by fluorescent color and/or intensity) from that of other fluorescent ink compositions of the plurality of fluorescent ink compositions) may be utilized for multicolor fluorescence by excitation with a single wavelength spectrum of light resulting in simultaneous detection of fluorescence of high quantum yield and with discrete peak emission spectra; (d) a fluorescent component that resists photobleaching (and therefore can be used for signal integration); and (e) a fluorescent component that is not susceptible to quenching.
Provided are fluorescent ink compositions comprising a fluorescent component that comprises functionalized fluorescent nanocrystals (e.g., a single type for print of a single fluorescent color, or a plurality of types for print of multicolor), an ink carrier, and a binder; and may further comprise other components such as one or more of a biocide, a surfactant, a defoamer, and the like. One or more fluorescent ink compositions are printed onto the surface of a substrate using a printer device. Following excitation of the printed surface with an appropriate excitation wavelength spectrum, each of the one or more fluorescent ink compositions printed on the surface will emit fluorescence of high quantum yield and with discrete peak emission. The fluorescent ink composition may be varied with respect to the intensity of fluorescence emission. Variables such as differences in intensity and in fluorescent color can enable an increase in the information that can be stored in a security mark, as well as increase the possible number and complexity of a security mark so as to prevent forgery. For example, the fluorescent ink composition can be varied by controlling the amount and type of functionalized fluorescent nanocrystals in the method of preparing the fluorescent ink composition. Thus, the fluorescent properties of the fluorescent ink composition, such as intensity and color, are sensitive to the functionalized fluorescent nanocrystals made apart thereof. A resultant advantage of the fluorescent ink compositions of the present invention is that they may be produced to have a greater degree of fluorescence (intensity) than previously known fluorescent jet compositions.
An additional advantage of the use of the fluorescent ink compositions according to the present invention relates to the use of a plurality of the fluorescent ink compositions in printing a desired pattern onto a surface. Each fluorescent ink composition may be comprised of a type of functionalized fluorescent nanocrystals capable of fluorescing a specific color. Thus, the plurality of fluorescent ink compositions maybe used to print a desired pattern which be excited to emit multicolor fluorescence which may be visible simultaneously in generating a specific, identifiable code (based on the emission spectra which can comprise both color and intensity) that can be used for purposes of verification or identification. For example, a plurality of fluorescent ink compositions may comprise a first ink composition capable of fluorescing red, a second ink composition capable of fluorescing blue, a third ink composition capable of fluorescing yellow, and a fourth composition capable of fluorescing green. This exemplary combination of a fluorescent ink compositions may be used to print currency with a security mark comprising a code (based on the number, or number and intensity, of colors) for verification purposes. Further, in a method of producing the fluorescent ink composition according to the present invention, by controlling the proportion of the components, precise control may be achieved with respect to the basic fluorescent properties of the resultant fluorescent ink composition.
The above and other objects, features, and advantages of the present invention will be apparent in the following Detailed Description of the Invention.